The Coronary Drug Project (CDP) is a prospective clinical trial designed to test the efficacy of several lipid-modifying drugs in the long-term treatment of coronary heart disease (CHD) and to study the natural history and clinical course of CHD. Males between the ages of 30 and 64, having a history of at least one ECG-documented myocardial infarction, were eligible for enrollment in the study. A total of 8,341 patients were enrolled and were randomly assigned to one of six treatment groups: Mixed conjugated equine estrogens (two dosage levels), clofibrate, dextrothyroxine, nicotinic acid, and placebo. As a consequence of careful periodic data review, the dextrothyroxine and both estrogen treatment groups were discontinued because of possible adverse drug effects. From November 1972 through February 1973, the CDP patients originally assigned to the first two discontinued treatment regimens who met a new set of eligibility requirements and gave their informed consent were enrolled in a second trial designed to test the efficacy of aspirin in the secondary prevention of CHD. A total of 1,147 patients were randomly assigned to a regimen of aspirin or placebo. Both studies are currently scheduled to terminate in February 1975.